1) Field of the Invention
The present invention relates to an aqueous ink-jet recording sheet. More particularly, the present invention relates to an aqueous ink-jet recording sheet capable of recording thereon images or letters in a brilliant color and in the form of a clear and exact dot, having an excellent water resistance, storage durability, dimensional stability, and easy handling property, and providing a satisfactory appearance even after a printing operation.
2) Description of the Related Arts
It is known that a recording sheet for forming a hard copy having a high quality, in accordance with an ink-jet recording method, is provided by coating a surface of a substrate sheet, for example, a paper sheet, with a coating composition comprising a pigment and a resinous binder, to form a coating layer which allows small drops of an aqueous ink jetted on the surface of the coating layer to rapidly penetrate the coating layer, at a right angle to the surface of the coating layer, to form clear images consisting of a number of dots on the coating layer surface.
Recent developments of the ink-jet recording system using an aqueous ink have enhanced the various performances of the printer, especially the printing speed, resolving power of the image formed and color-brightness, and thus the recording sheet for the ink-jet recording system must have a greatly improved ink-absorbing speed, ink-absorbing capability, and allow a regular penetration of the ink.
For example, Japanese Unexamined Patent Publication No. 62-158084 discloses a method of producing an ink-jet recording material in which specific fine synthetic silica particles are contained in a coating layer. From this Japanese publication, it is known that the specific fine silica particles exhibit a high ink-absorbing speed, a large ink-absorbing amount, a small spread of absorbed ink, a circular-shaped ink-dot, and no influence on the color development of the ink, and thus are excellent as an ink-absorbing agent for the ink-jet recording sheet.
The coating layer comprising the above-mentioned specific fine silica particles is disadvantageous in that, when the aqueous ink is absorbed in the coating layer, a coloring material in the aqueous ink deeply penetrates, together with an aqueous medium in which the coloring material is dissolved or dispersed, the coating layer, and thus the resultant images on the coating layer surface exhibit an unsatisfactory optical density and brightness. Accordingly, to form clear and bright color images on the coating layer surface, the coloring material in the ink must be maintained in the surface portion of the coating layer and only the aqueous medium of the ink allowed to be absorbed by the entire coating layer.
The ink-jet recording sheet can be easily printed in the same way as usual paper sheets, and thus the ink-jet recording sheet is expected to be widely utilized in various fields in which conventional fine paper sheets and coated paper sheets are used. Accordingly, the ink-jet recording sheet must provide not only an appearance similar to a conventional fine paper sheet or coated paper sheet but also an excellent aptitude for practical utilization.
For example, the ink-jet recording sheet must meet the requirements of a satisfactory writing ability with a ball-point pen or pencil, a required touch and slippage, a low friction between recording sheets, and an easy handling when supplied as a paper roll or cut paper sheets to a printer.
It is known that a conventional ink-jet recording sheet having a coating layer in an amount of 10 g/m.sup.2 or more and comprising, as a principal component, fine silica particles, is disadvantageous in that it is difficult to write thereon with a ball-point pen or pencil, the touch is not satisfactory, the slippage is poor or too high, the friction between the paper sheets is too high or too low, and the feed and delivery of the sheets in the printer is difficult.
Also, when aqueous ink images are formed on a coating layer comprising the fine synthetic silica particles, the aqueous ink is fixed on and within the coating layer. The aqueous ink and the synthetic silica particles and other component of the coating layer are hydrophilic, and thus the ink images on the coating layer have a poor water resistance.
To enhance the water resistance of the ink images on the ink-jet recording sheet, it has been recently attempted to add a cationic surface-active agent and/or a cationic, water-soluble polymer to the coating layer. It is already known that color images formed by a water-soluble dye can be fixed with the cationic surface-active agent or cationic water-soluble polymer, and that the fixed color images exhibit an enhanced water resistance.
For example, many attempts have been made to contain in the coating layer for receiving ink images a water resisting agent consisting of a polycationic electrolytic polymer, for example, polyvinyl pyridium bromide or polyethylene imine, as disclosed in Japanese Unexamined Patent Publication No. 56-84992, dimethyldiallyl ammonium chloride as disclosed in Japanese Unexamined Patent Publication No. 59-20696, polyethylene imine-organic acid salts as disclosed in Japanese Unexamined Patent Publication No. 59-198186, polyalkylene polyamine dicyandiamide ammonium salt-condensation products as disclosed in Japanese Unexamined Patent Publication No. 60-49990, or polyethyleneimine-quaternary ammonium compounds as disclosed in Japanese Unexamined Patent Publication No. 60-76386.
The images comprising an ink and formed on a coating layer comprising a pigment, a binder and other additives show an improved water resistance thereof when the above-mentioned cationic compounds are added to the coating layer, but the effect of the conventional cationic compound on the enhancement of the water resistance is not satisfactory, and the addition of the conventional cationic compounds additionally causes a lowering of the storage durability of the ink images.
Further, attempts have been made to improve the ink-absorption of the ink-jet recording sheet. For example, Japanese Examined Patent Publication No. 60-27588 discloses an ink-jet recording sheet composed of a non-sized paper sheet or a coated paper sheet in which a substantially non-sized paper sheet is coated with an absorbing layer of silica. Also, Japanese Examined Patent Publication No. 63-65037 discloses an ink-jet recording sheet having an enhanced color image-forming property and composed of a non-water-absorbing substrate sheet, for example, a highly sized paper sheet or a plastic film, and a high water-absorbing coating layer comprising, as a principal component, silica particles.
Among the above-mentioned various types of conventional ink-jet recording sheets, the ink-jet recording sheet having a water-absorbing substrate sheet is considered superior, in view of the balance of the cost with the touch and required properties. This type of recording sheet must have a high ink-absorbing property such that the ink is rapidly absorbed into the inside of the substrate sheet and the resultant ink images received on the recording sheet appear to be dry, and a high resistance to the tendency of curling and cockling. Generally, since the tendency of curling and cockling is increased with an increase in the ink absorption of the recording sheet, the requirement for an enhancing of the ink-absorption of the recording sheet is contradictory to the requirement for preventing the tendency of curling and cockling. For example, where the ink-absorbing speed of the recording sheet is enhanced by using a low sized paper sheet as a substrate sheet, the resultant recording sheet allows the ink to rapidly and deeply penetrate into the substrate sheet, and thus curling and undulations are easily formed in the recording sheet.
To eliminate the above-mentioned disadvantages, an ink-absorbing coating layer is formed on a highly-sized substrate paper sheet. In this recording sheet, the ink is rapidly absorbed in the coating layer and the penetration of the ink in the substrate sheet is restricted. Nevertheless, when the ink is absorbed in a large amount in excess of the upper limit of the ink-absorbing capacity of the coating layer, an undesirable bleeding of the absorbed ink occurs. Also, the above-mentioned absorption of ink in the coating layer results in a generation of curling or corrugations in the recording sheet. Further, this type of recording sheet is accompanied with an increased cost.
Japanese Examined Patent Publication No. 63-52588 discloses an ink-jet recording sheet comprising a paper sheet in which glass fibers and a large amount of water-absorbing pigment are mixed with a cellulose pulp, to lower the bonding strength of the cellulose pulp fibers to each other and to prevent the formation of curling and corrugations in the paper sheet when the ink is absorbed.
Nevertheless, the addition of glass fibers makes the handling of the resultant sheet difficult and causes a lowering of the mechanical strength of the resultant sheet, and thus the practical utility of the resultant recording sheet is doubtful.
Conventional paper sheets having a high dimensional stability even when the moisture content of the sheets and the humidity of the ambient atmosphere are fluctuated are known, and are utilized as paper sheets for optical character reader or NIP (non-impact printing) paper sheets.
Nevertheless, it is not conventionally known how to prevent the formation of curling and cockling on the ink-jet recording sheets, and thus ink-jet recording sheets free from the creation of curling and cockling therein are not produced.